1

1.1 Beginning of Cement "Cement" as Portland Cement was first made in ashaft kiln using dry process and later in rotary kilns.

That "Slaked" lime hardens with water was wellknown and was used as "Mortar" with sand inconstruction industry before the advent of cement.

Some "Natural rocks" contained all ingredients likeCaO, SiO2, Al2O3 and Fe2O3 in approximately rightproportions so that they did not need any additions andwhen ground calcined and sintered in a kiln producedclinker which when ground with 5% gypsum producedwhat has come to be known as "Portland Cement".

Cement has hydraulic properties like slaked limeand hardens when mixed with water. Compressivestrength increases in time and reaches its practical toplimit after 28 days.

Mixing crushed stone, sand, cement and watermakes "Concrete". When hardened it is like rock andhence is called "Synthetic rock". It has similar propertiesof high compressive but low tensile strengths.

When concrete is poured around steel it becomesReinforced Cement Concrete - popularly known asRCC - and has high tensile strength also.

RCC has revolutionized construction industry andit is well nigh unimaginable to construct roads, dams,skyscrapers and silos and many other large and heavybuildings for residential or for industrial purpose withoutRCC.

Cement the main strength giving and bindingingredients is thus an all-important part of RCC andthus plays a vital role in the progress and developmentof a nation.

At present there is no substitute for cement.Hence, it will continue to play an all-important role inconstruction industry.

Yardsticks like inter alia per capita consumption ofsteel, power and cement are used to indicate state ofdevelopment of countries. Advanced and developedcountries have per capita consumption of cement of400-500 kgs. As against it, in India per capitaconsumption of cement is only about 195/200 kgs.

1.2 Making CementCement Industry started in a very small way, first asshaft kilns using dry process.

When it was found that the proper composition ofraw mix required to making good quality clinker almostalways needed, additions or correcting materials tocompensate for constituents like Silica, Alumina andIron Oxides, it became necessary to "Blend" theconstituents after "Grinding".

Blending was then more convenient in wet stage inthe form of slurry.

By this time Rotary Kiln had come to be used tomake Cement. It would conveniently receive slurry aswell. Thus process of cement making changed fromDry to Wet.

1.3 Dry to Wet to DryProcess of Manufacture

Wet Cement plants continued to grow in number andsize and wet process was the predominant process ofmanufacture of cement till 1950 or so. It continued tobe the dominant process in India for another twodecades.

CHAPTER 1

OVERVIEW OF MANUFACTURE OF CEMENT

Section 1. BASICS2

Wet process was simple and required less processcontrol and instrumentation and manpower. But itconsumed a large quantity of water and also heat energyin drying the slurry.

As fuel costs rose, alternative processes wereinvestigated to reduce water content of slurry andthereby fuel consumption.

1.4 Semi-dry ProcessThus came into use 'semi-dry' process which neededonly 8-10% water compared to 35-36% for wet slurry.Raw materials were ground and blended dry. Waterwas added to dry raw mix in a revolving pan to makenodules. The nodules were dried on a travelling gratepreheater before feeding them to a rotary kiln or to ashaft kiln.

1.4.1 V. S. KilnsVertical Shaft Kilns also came to be developed forcapacities up to 300 tpd (in Europe). They needed lowvolatile fuels like coke breeze.

1.5 Dry ProcessWhen wet process plants had reached their peakcapacity of 750-1000 tpd, developments in processesand machinery took place that once again changedthe course of cement making.

1.6 Suspension Preheaters In early 50s of the 20th Century, an epoch makingconcept was developed - that of Suspension Preheater.

The suspension preheater, with rotary kiln and gratecooler formed the heart of the cement plant and 'dryprocess' came to be adopted fast and number of wetprocess plants and semi dry process plants declined.

Even in India, which had predominance in wetprocess plants in numbers and capacity as late assixties, the percentage has decreased to less than 3% now.

1.7 Vertical Mills and CalcinersOther epoch making developments took place in 70s.They were using Vertical mills for grinding rawmaterials and coal and development of calciners whichcalcined raw meal before it entered the kiln. With acalciner, output of the same kiln could be increased byabout two and a half times.

Vertical mills have also came to be used for grindingcement clinker and slag.

Roller Presses are also now part of grinding syatemsThese two developments gave tremendous boost

to the size of the plant and also to economies in powerconsumption.

1.8 Benchmarks in Manufacture of CementProgress in making cement described above has beenshown in Table 1.1.1 and pictorially in Figs. 1.1.1(a),1.1.1(b) and 1.1.2

1.9 Fuel and Power ConsumptionFuel consumption was also steadily brought down from1500 Kcal/kg clinker for wet process kilns to 800 Kcal/kg clinker for dry process kilns with 4 stage preheaterand grate cooler. This has further come down to lessthan 700 Kcal/kg by using 6 stage preheaters.

Power Consumption has come down to 85-90 Kwh/Ton.

1.10 Green CementCement industry is now conscious of the emission ofgreen house gases emitted in the process of makingcement. Green house gases cause global warming.

Cement Industry is therefore taking steps to reducethese emissions. Therefore it makes ' blended cements'like Pozzolana cement (PPC) and slag cement ( BFSC)wherever possible. It also uses Alternate fuels toreduce emissions arising out of combustion.

Cement Industry in countries, like India which wereshort of power, had begun to install captive power plantsto ensure continuity of power. This is taken one stepfurther by using waste heat in exit gases from kiln andcooler to generate power.

These new developments have been dealt with inthis new edition to the extent cement plant engineersare commonly required to deal with them.

1.11 Differences in ProcessesDifferences in various processes of making cementand equipment used therein have been brought outin Tables 1.1.2 and 1.1.3 and in Fig. 1.1.2.

1.12 Size of the PlantFrom beginning with a 20-30 t.p.d. capacity, individualkilns have gone up in size upto 7500-10,000 tpdcapacity. Plants of 3-4 mtpa capacity in one place havealso become common.

Overview of Manufacture of Cement 3

Fig.

1.1

.1 (a

)B

ench

mar

ks in

man

ufac

ture

of c

emen

t.

Section 1. BASICS4

Fig.

1.1

.1 (b

)B

ench

mar

ks in

man

ufac

ture

of c

emen

t.

Overview of Manufacture of Cement 5

BenchmarksKilns Shaft kiln-Rotary Kiln - Wet - Long dry - Short - Semi dry and Dry

preheater kilnsCoolers Rotary Cooler – planetary cooler;

Traveling and Reciprocating grate coolers;Static grate, pendulum coolers and cross bar coolers

Preheaters Calciners in wet kilns, traveling grate preheaters;Suspension preheaters 4 – 6 stages.

Mills Ball and tube mills – mills with slide shoe bearings;Vertical ring and roller mills;

Roller press and ball mill combinations;Horizontal Roller mill

Blending Slurry blending – slurry mixer;Air merge blending – batch and continuous

Classifiers Wet classifiers;Dry classifiers- grit separators, mechanical air separators;

High efficiency separatorsCrushers Jaw, Hammer, Roll and Impact, mobile and semi mobile

crushersPacking machines Stationary – rotary – rotary electronic

Calciners In and off line; spouted bed, fluidized bed and many othersDespatches Mechanized-automated loading of bagged cement;

Bulk cement by road and rail and seaDust collectors Cyclones; poly and multiclones;

Bag filters- glass bag filters;Gravel Bed filters;

Electrostatic Precipitators

Table 1.1.1 Benchmarks in manufacture of cement.

The trend even in India is to go in for large plantsto avail of economies of scale which can be achievedby using machineries (Vertical Mills, ESPS, etc.) whichare suitable for and affordable by large plants.

The days of small plants and particularly those ofVSKs are over and they would not be considered forany future cement projects.

1.13 Sizes of Cement Plants in IndiaEven in conventional plants using rotary kilns, therewas a distinction.

1. Mini Plants of capacity - 300 tpd - to start with-but expanded to 1000-1200 tpd capacity byinstalling calciners

2. Large Plants of capacity - 600 tpd and aboveearlier, 3000 tpd and now to 10000 tpd above

Thus as far as process and types of machinery usedare concerned, the distinctions between large and smallplants have almost blurred. New projects are almostinvariably for dry process plants of +1.0 MTPAcapacity.

1.13.1 Large Cement PlantsThere are already about 183 cement plants of +1MTPA capacity. Average capacity per plant is 1.7mtpa. Total installed capacity is 480 mtpa.

Section 1. BASICS6

Tab

le 1

.1.2

Con

td...

.

Tabl

e 1.

1.2

Bas

ic D

iffer

ence

s be

twee

n Va

rious

Pro

cess

es o

f Man

ufac

ture

of C

emen

t.

Sr. N

o.S

ectio

nW

etS

emi

Wet

Sem

i D

ryD

ryD

ry w

ithca

lcin

er

(A)

Raw

mat

eria

l pr

epar

atio

n

1.C

rush

ing

Com

mon

to a

ll pr

oces

ses;

mac

hine

ry s

elec

ted

depe

nds

on s

ize

of p

lant

and

pro

perti

es o

f sto

ne

2.G

rindi

ngW

et g

rindi

ng in

bal

l mill

sD

ry g

rindi

ng; d

ryin

g du

ring

grin

ding

; mos

tly in

clo

sed

som

etim

es i

n cl

osed

circ

uit;

circ

uit;

usin

g ki

ln g

ases

for

dry

ing.

prod

uct s

lurr

y w

ith 3

5-40

% w

ater

ball

mill

s, v

ertic

al m

ills

and

rolle

r pr

ess

in h

ybrid

grin

ding

prod

uct d

ry r

aw m

eal w

ith le

ss th

an 1

% m

oist

ure

3.H

omog

enis

ing

wet

, air

and

mec

hani

cal a

gita

tion

fluid

isat

ion

tech

inqu

es fo

r bl

endi

ng d

ry p

ulve

rised

raw

mea

lpr

oduc

t ble

nded

slu

rry

wth

35

% w

ater

batc

h or

con

tinuo

us b

lend

ing;

cont

inuo

us b

lend

ing

will

be p

rece

ded

by p

reho

mog

enis

ing

in s

tock

pile

s;pr

oduc

t dry

ble

nded

raw

mix

4.K

iln fe

edsl

urry

with

~ 3

4 %

eith

er e

xtru

ded

pelle

ts w

ith 8

-10%

dry

pulv

eris

ed ra

w m

eal f

ed th

roug

hm

oist

ure

pelle

ts w

ith 1

5%%

moi

stur

evo

lum

etric

or g

ravi

met

ic fe

eder

ssy

nchr

onis

ed w

ithm

oist

ure

or,

dry

kiln

raw

mea

l drie

din

flas

h dr

yer

Overview of Manufacture of Cement 7

Sr. N

o.S

ectio

nW

etS

emi

Wet

Sem

i D

ryD

ry

Dry

with

c

alci

ner

(B)

Pyro

proc

essi

ng

1.dr

ying

in k

ilndr

ying

of

pel

lets

dryi

ng o

f nod

ules

preh

eatin

g in

kiln

p

rehe

atin

g ou

tsid

e2.

preh

eatin

gin

kiln

in tr

avel

ling

grat

e pr

ehea

ter

for

long

dry

kiln

k

iln in

dry

pre

heat

erpr

ehea

ting

in p

rehe

ater

k

ilns;

mos

tly

cyc

lone

pre

heat

ers

3.ca

lcin

ing

in k

ilndi

ssoc

iatio

n of

CO

2 be

ginn

ing

at ~

600

oC

and

com

plet

ing

at ~

950

oC

in k

ilnla

rgel

y in

kiln

partl

y in

pre

heat

er

alm

ost

tota

lly o

utba

lanc

e in

kiln

o

f kiln

in c

alci

ner

4.si

nter

ing

f

orm

atio

n of

clin

ker

at 1

250-

1450

oC

in k

iln

5.cl

inke

r co

olin

g

co

olin

g of

clin

ker

to 6

5- 1

50 o

C c

omm

on to

all

proc

esse

s

now

mos

tly r

ecip

roca

ting

grat

e co

oler

s w

ith v

aria

tions

like

sta

tic g

rate

, co

ntro

lled

flow

and

pen

dulu

mty

pe a

re u

sed

Section 1. BASICS8

Sr. N

o.Pr

oces

sW

etSe

mi W

etSe

mi D

ryDr

yD

ry w

ith C

alci

ner

1.

crus

hing

com

mon

to a

lltw

o st

age

crus

hing

for b

all m

ills;

sin

gle

stag

e cr

ushi

ng fo

r Ver

tical

mill

s an

d R

olle

r Pre

sses

Jaw

cru

sher

– H

amm

er c

rush

er c

ombi

natio

n fo

r tw

o st

age

crus

hing

Impa

ctor

s –

sing

le o

r tw

o st

age

for s

ingl

e st

age

crus

hing

sem

i mob

ile, m

obile

cru

sher

s fo

r lar

ge p

lant

s

2.

grin

ding

bal

l mill

ope

n or

clo

sed

circ

uit

ba

ll m

ills-

air-

swep

t or b

ucke

t ele

vato

r in

clos

ed c

ircui

t

a

utog

enou

s m

ill

w

ith c

onve

ntio

nal o

r hig

h ef

ficie

ncy

sepa

rato

rs

v

ertic

al ro

ller m

ills

- vrm

s w

ith e

xter

nal c

ircui

t

an

d hi

gh e

ffici

ency

sep

arat

ors

ro

ller p

ress

and

bal

l mill

in v

ario

us c

ombi

natio

ns

3.

preh

omog

enis

ing

st

acke

r rec

laim

er s

yste

ms

used

for p

reho

mog

enis

ing

of li

mes

tone

s an

d co

al d

urin

g bu

ildin

g up

of a

nd

ext

ract

ion

from

sto

ck p

iles

4.

hom

ogen

isin

gpn

eum

atic

and

mec

hani

cal a

gita

tion

b

lend

ing

syst

ems

base

don

‘flui

disi

ng’ t

echn

ique

s

slu

rry

mix

ers

b

atch

and

con

tinuo

us b

lend

ing

syst

ems

5.

kiln

feed

met

erin

g of

slu

rry

filte

r pre

ss a

ndno

dulis

er w

ithw

eigh

feed

er o

r sol

ids

flow

met

ers

sync

hron

ised

with

disa

gglo

mer

ator

varia

ble

spee

d

w

ith p

refe

eder

ski

lnan

d fla

sh d

ryer

or fi

lter p

ress

and

extru

der

Tab

le 1

.1.3

Con

td...

.

Tabl

e 1.

1.3

Diff

eren

t typ

es o

f Equ

ipm

ent u

sed

in v

ario

us P

roce

sses

.al

so s

ee F

ig. 1

.1.2

Overview of Manufacture of Cement 9

Sr. N

o.Pr

oces

sW

etSe

mi W

etSe

mi D

ryDr

yD

ry w

ith C

alci

ner

6.

preh

eate

rca

lcin

ator

trave

lling

grat

etra

vellin

g gr

ate

susp

ensi

on p

rehe

ater

or s

haft

kiln

preh

eate

r or

preh

eate

r or

susp

ensi

onsu

spen

sion

preh

eate

rpr

ehea

ter o

rsh

aft k

iln

7.

calc

inin

gca

lcin

ers

in o

r off

line

8.

clin

kerin

gro

tary

long

kiln

rota

ry s

hort

kiln

rota

ry s

hort

kiln

rota

ry s

hort

kiln

rota

ry s

hort

kiln

shaf

t kiln

rota

ry lo

ng k

iln

9.

clin

ker c

oolin

g c

omm

on to

all;

rota

ry a

nd p

lane

tary

coo

lers

for s

mal

l pla

nts

– no

w a

lmos

t dis

card

ed;

rec

ipro

catin

g gr

ate

cool

ers

of v

ario

us d

esig

ns li

ke s

tatic

gra

te, c

ontro

lled

flow

gra

te

p

endu

lum

coo

ler

cros

s ba

r coo

ler e

tc.

Follo

win

g pr

oces

ses

are

com

mon

to c

emen

t pla

nts

of a

ll ty

pes

diffe

ring

in s

cale

acc

ordi

ng

to

size

of p

lant

10.

coal

grin

ding

ba

ll or

ver

tical

mill

s w

ith d

ryin

g fa

cilit

ies

11.

cem

ent g

rindi

ng

b

all m

ills

in c

lose

d ci

rcui

t and

hig

h ef

ficie

ncy

sepa

rato

rs

verti

cal m

ills

with

ext

erna

l circ

uit a

nd h

igh

effic

ienc

y se

para

tors

ro

ller p

ress

and

bal

l mill

and

hig

h ef

ficie

ncy

sepa

rato

rs in

a n

umbe

r of w

ays

12.

cem

ent p

acki

ng

rota

ry o

r sta

tiona

ry p

acke

rs to

pac

k ce

men

t in

jute

/pap

er b

ags

loos

e ce

men

t

ce

men

t sen

t in

bulk

in b

ulk

carr

iers

by

road

bys

hips

etc

.

13.

cem

ent

by

road

or r

ail o

f bag

ged

cem

ent u

sing

sem

i or f

ully

mec

hani

sed

load

ing

mac

hine

sde

spat

ches

a

lso

by s

hip

load

s fo

r exp

ort

Section 1. BASICS10

Fig.

1.1

.2 E

quip

men

t use

d in

man

ufac

ture

of c

emen

t.